The purpose of this study is to determine the pharmacokinetic disposition of IV cidofovir in patients with varying degrees of renal function and patients undergoing hemodialysis or continuous ambulatory peritoneal dialysis. The GCRC will be utilized to conduct this study by assisting in drug administratign (intravenous cidofovir and normal saline, as well as oral concomittant medications), blood and urine collections, and monitoring for patient safety.